Predictive Maintenance, or PdM, programs in industrial plants are frequently implemented by assigning a technician to use portable instrumentation, such as a vibration analyzer, an ultrasonic gun, and/or an IR camera, along a predetermined route to collect data related to the operation of the equipment on this route. This information, in turn, may then be used to diagnose problems or potential problems associated with the health and/or operation of the equipment.
For example, a PdM program may include a technician carrying a vibration analyzer to each machine located along a defined route. Upon reaching a particular machine to be analyzed, a vibration sensor, such as an accelerometer, is physically coupled to the machine at one or more measurement locations. Frequently, the data to be acquired at each measurement location is specified as part of the route instructions. The vibration sensor and analyzer then receive vibration data from the measurement locations, and may output this information on a display of the analyzer.
There are a number of options for coupling vibration sensors to the machinery, and a variety of sensor configurations which have been employed. Two factors which have a significant impact on the sensor and mounting method selected are the speed and ease of mounting and the resulting frequency response of the mounted vibration sensor. Accelerometers are the most often used vibration sensors because they are affordable and relatively easy to handle when mounting and removing them from machines. It is common for these sensors to be calibrated to have a flat frequency response from 0 to 10 kHz. This wide frequency response is quite acceptable for machine measurements; however, it is generally only achieved when significant attention is given to its mounting method. It is widely recognized that stud mounting the sensor to a flat machined surface provides the best results, approaching that which is used during the calibration procedure. However this is hard to achieve in an industrial environment in which a portable sensor is being used to monitor all types of machine designs. It may be desirable for a single operator to collect data from 50 or more machines in one 8-hour shift, due to the desire to reduce the cost of data collection by reducing the man-hours required. The cost of the data collection labor will greatly exceed the investment made in the PdM analyzers and software over time. One solution to gain speed is to hand-hold the sensor against the machine or to mount the accelerometer using a magnet that is stud-mounted to the sensor housing. These methods are quick; however, they often result in lowering the flat sensitivity region to less than 1 kHz. This frequency response can be improved by using mounting pads that are fixed to a flat surface on the machine housing, such as with a special epoxy. These pads provide an additional benefit in that they insure that the data is collected from exactly the same position on the machine on every monitoring cycle regardless of who is collecting the data. Rare earth magnets attached to the accelerometers and placed on the pad may increase the flat frequency response achieved to 2-4 kHz. Stud mounting the sensor to these pads can result in a flat frequency region which is close to that achieved during calibration.
Single axis and triaxial accelerometers have become popular choices for portable PdM applications. Single axis accelerometers have the broadest use because of their smaller size, lower cost, and ease of use with magnetic mounting options, and because many early instruments could only collect a single channel of data. The triaxial accelerometer has also become popular since the advent of multi-channel data collection instruments because it allows 3 channels of data to be collected at one time. It is often desirable to collect data in three spatial axes (x, y, and z directions) at each bearing housing on the machine. To accomplish this with a single axis sensor, the operator must position the sensor at each of the three orientations and make a measurement. Clearly, this increases the data collection time three-fold, which is not desirable. However, the use of triaxial accelerometers dictates the use of mounting pads in order to get repeatable data collected. It is not desirable for the operator to have to remember how the sensor should be oriented at each location, or to experiment with its placement to try and repeat the orientation used in the previous application. To insure repeatable positioning of a triaxial accelerometer, the sensors and the mounting pads are manufactured with mating alignment structures. The most common practice followed by the manufacturers of triaxial accelerometers is to use a center mounting screen and a cylindrical alignment pin built into the mating surface of the sensor and positioned to be aligned with the axis of one of the three internal sensors. This alignment peg can be used to mate with a hole or notch in the mounting pad to guarantee repeatable positioning of the sensor on repeated applications.
This option of using triaxial accelerometers and mounting pads with a center thread hold and an alignment notch is a long standing practice of more than ten years. The sensors are typically gripped in one hand and aligned to the block, while the other hand of the user holds a ball driver to thread the mounting screw into the center hole of the pad. There has been virtually no change in the mechanical design for mounting the sensor to these pads for more than a decade.
As previously discussed, the vibration sensor is typically coupled to a mounting pad that has been previously fixed to the machine that is to be analyzed, and the operator, or user, mounts the vibration sensor to the mounting pad to begin the analysis. FIG. 1 illustrates a conventional triaxial accelerometer and mounting pad used in such a vibration measurement. As illustrated in FIG. 1, the mounting pad 101 is fixed to an outer surface of a machine 102 such as would be found in a factory. As several different types of machines, appliances, etc., may be monitored in a vibration measurement, for convenience of illustration, only a surface portion of the machine 102 upon which the mounting pad 101 is fixed is illustrated in this drawing. Also, the mounting pad 101 may be fixed at several different locations on the machine 102, such as on a side, top, or bottom surface.
Referring to FIG. 1, the mounting pad 101 is provided with an alignment recess 103 to aid in the proper orientation of the triaxial accelerometer, referred to generically herein as a vibration sensor 104. As indicated in FIG. 101, the vibration sensor 104 is designed such that movement in each of the X, Y, and Z axes is measured, the proper orientation of the vibration sensor 104 is achieved through mounting the vibration sensor 104 such that an alignment pin 105 of the vibration sensor 104 is received in the alignment recess 103 of the mounting pad 101. Thus, the mounting pad 101 is previously fixed to the machine 102 with the alignment recess 103 at the proper position to receive the alignment pin 105 at the proper orientation. The vibration sensor 104 is typically provided with a data cable 110 to transmit the sensed vibration data to a data acquisition unit (not illustrated).
Upon aligning the vibration sensor 104 according to the proper placement of the alignment pin 105 in the alignment recess 103, an attachment screw 106 is used to fix the vibration sensor 104 to the mounting pad 101. The attachment screw 106 is passed through a threaded through hole 107 of the vibration sensor 104 into a threaded receiving aperture 108 of the mounting pad 108. The attachment screw 106 typically has a hex socket 109 so that the attachment screw 106 may be driven by a hex key or driver. The attachment screw 106 is driven, with the threads of the attachment screw moving along the threaded receiving aperture 108, until the head of the attachment screw 106 is flush against the outer surface of the vibration sensor 104, at which point the vibration sensor 104 is fixed in position.
The conventional method of mounting the vibration sensor 104 on a mounting pad 101 typically requires a user to employ both hands to perform the sensor mounting operation. FIG. 2 illustrates a conventional method of mounting the vibration sensor 104 onto the mounting pad 101. As illustrated in FIG. 2, the user holds the vibration sensor 104 in one hand, both to move the vibration sensor 104 into the proper orientation according to the alignment pin 105 being received in the alignment recess 103, and also to hold the vibration sensor 104 in position while the attachment screw 106 is driven by a driver 201 held in the user's other hand. As illustrated in FIG. 2, the driver 201 typically is provided with a hex key 202 on a driving end thereof. Even if the user advances the attachment screw 106 far enough through the threaded through hole 107 such that the end of the attachment screw 106 extends out of the surface of the vibration sensor 104 which faces the machine 102, so that threading of the attachment screw 106 into the receiving aperture 108 may be started before the proper orientation of the vibration sensor 104, both hands of the user are still required so that both the vibration sensor 104 and the driver 201 can both be held to perform such an operation.
As previously discussed, the mounting pad 101 may be provided on various different surfaces of the machine 102, including the underside, recessed portions, etc., that are very difficult for a user to reach with both hands to perform the simultaneous holding of the proper orientation of the vibration sensor 104 and the driving of the attachment screw 106. Also, even in a situation in which access to the mounting pad 101 is relatively easy, it is an inconvenience for the user to have to use both hands to perform the mounting operation, as one hand could be otherwise used to operate a data collecting device, cellular phone, etc.
As discussed above, for a variety of reasons, there is a desire for a mounting mechanism to improve the safety and speed of this approach. One concern is that the operator typically has an instrument that usually needs to be controlled while the operator is attaching the sensor to locations which are often not convenient. Additionally, the machine surface may be hot or coated with undesirable chemicals. Placing one's hands within ½ inch of the machine housing, possibly near to moving parts, is not desirable from a safety perspective. The sensor or ball driver can also represent a hazard if dropped or made to contact moving parts of the machine. Thus, there is a desire to improve the method of attaching a triaxial sensor to an indexed mounting pad using a one-hand operation that is faster and safer than those currently available.
Another consideration in improving the speed of mounting is dependent on how the sensor and attachment tool are carried when moving from one measurement location to the next, on the same or a different machine. The operator is often moving about in rather restrictive areas in which both hands are needed to prevent potential accidents, which is problematic when the operator is carrying the sensor and mounting equipment. Also, the vibration sensor is a delicate and expensive component, and the vibration sensor may be in jeopardy of being damaged due to inadvertent contact with various hard surfaces on the way to being mounted, or due to being dropped by the user during the operation. Therefore, a device that would provide support and protection for the vibration sensor during the mounting process would be of value.
The improvement in operator safety is always of utmost importance in any industrial operation. The significance of increasing the speed and ease of mounting a sensor to the machine may not be appreciated unless it is realized that an operator may perform this operation literally hundreds of times during a typical measurement survey. The cumbersome nature of mounting triaxial accelerometers has been one of the factors which has slowed their adoption by industry even though their use offers many advantages.